1. Field of the Invention
The present invention relates to a method and to apparatus for using a rotorcraft turboshaft engine in an optimized performance envelope that is different from the performance envelope initially authorized for the engine.
2. Description of the Related Art
Most rotorcraft presently being built have one or two free turbine engines. Power is taken from a low-pressure turbine referred to as a “free” turbine, which is mechanically independent of the compressor assembly and the high-pressure stage of the engine, which stage comprises a high-pressure turbine in particular. The free turbine of an engine generally rotates at a speed in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a stepdown gearbox is needed for its connection to the main rotor of the rotorcraft, since its speed of rotation lies substantially in the range 200 rpm to 400 rpm: this is known as the main transmission gearbox.
The temperature limitations of an engine and the torque limitations of a main gearbox serve to define a performance envelope covering two normal utilization ratings for an engine arranged on a single- or twin-engined rotorcraft;                takeoff rating corresponding to a level of torque for the gearbox and to a level of temperature for the engine that can be accepted for a limited length of time without significant degradation: this is known as maximum takeoff power (PMD) and can be used for five minutes; and        maximum continuous rating during which, at no time, are the capabilities of the gearbox exceeded or the capabilities of the engine exceeding relating to the maximum temperature that can be accepted continuously ahead of the high pressure blades of the first stage of the turbine: this is maximum continuous power (PMC) and it can be used without time limit, corresponding to about 90% of PMD.        
On a twin-engined rotorcraft, the performance envelope also covers emergency ratings, for use only with one engine inoperative:                the first emergency rating during which the capabilities of the inlet stages of the gearbox and the temperature capabilities of the engine are used to the maximum: this rating can be used for a maximum of thirty consecutive seconds, and on three occasions in any one flight, it is equal to about 112% to 120% of PMD and is referred to in the art as super-emergency power (PSU). The use of PSU requires the engine to be removed and overhauled;        the second emergency rating in which the capabilities of the inlet stages of the gearbox and the temperature capabilities of the engine are used to a very great extent: this rating is equal to approximately 105% to 110% of PMD, it can be used for a maximum of two consecutive minutes, and it is referred to as maximum emergency power (PMU)        the third emergency rating during which the capabilities of the inlet stages of the gearbox and the temperature capabilities of the engine are used without inflicting damage: this is the intermediate emergency power equal to the PMD that can be used with one engine inoperative and can be used for the remainder of the flight, being referred to as PIU.        
Consequently, the temperature and mechanical constraints, and above all the phenomenon of turbine blade creep, lead to the engine being degraded to a greater or lesser extent depending on the rating. To guarantee safety in flight and to obtain high performance, it is therefore essential to determine the maximum amount of damage that is acceptable for an engine.
Thereafter, the total service life of the engine is evaluated. In practice, this reduces to defining a maximum number of flying hours that the engine can perform between overhauls (or since its first use, depending on circumstances), and is referred to as time between overhauls (TBO). Once TBO has been reached, the engine is removed and overhauled.
For convenience in the text below, the term “most recent overhaul of the engine” is used to designate either first use of the engine or else the genuine most recent overhaul thereof.
Furthermore, in order for a rotorcraft to obtain authorization to fly in any given country, it is required that the performance envelope and the TBO of the engine(s) of the rotorcraft be certified by the official services in the country in question for a precise utilization spectrum. Such authorization is therefore achieved only after complete certification testing that is very expensive.
Since such complete certification tests of an engine are performed specifically to justify a performance envelope associated with a TBO, it is not possible to use the engine with a performance envelope different from the initially-authorized performance envelope, without performing new complete certification tests that are very expensive.
By way of example, it is found that the performance envelope of the type described above, associated with a TBO of about 2500 hours, corresponds to a utilization spectrum of a type that matches most civil applications. Nevertheless, for military applications or for certain particular missions, e.g. a rescue mission requiring winching into a helicopter, such an envelope can be insufficient. While on the contrary, in other situations, the envelope might be overdimensioned.
To remedy this problem, one solution would be to manufacture different engines dedicated to specific applications for a given rotorcraft airframe. However, given the cost of development, certification, and integration, that solution can be seen to be unsatisfactory. Production levels must be high in order to recover adequately the investment involved. That goes against the desired principle whereby a special performance engine is used on an existing rotorcraft in order to satisfy a particular need, which by its very nature implies short production runs.